Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K3/00—Apparatus or processes for manufacturing printed circuits
  • H05K3/0011—Working of insulating substrates or insulating layers
  • H05K3/0017—Etching of the substrate by chemical or physical means
  • H05K3/0026—Etching of the substrate by chemical or physical means by laser ablation
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/36—Removing material
  • B23K26/38—Removing material by boring or cutting
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
  • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
  • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
  • B23K26/36—Removing material
  • B23K26/38—Removing material by boring or cutting
  • B23K26/382—Removing material by boring or cutting by boring
  • B23K26/389—Removing material by boring or cutting by boring of fluid openings, e.g. nozzles, jets
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K2201/00—Indexing scheme relating to printed circuits covered by H05K1/00
  • H05K2201/09—Shape and layout
  • H05K2201/09009—Substrate related
  • H05K2201/09063—Holes or slots in insulating substrate not used for electrical connections

Definitions

• the technical field relates to lasers and more particularly to methods for employing laser beams to rapidly process holes in various specimen materials.
• Trepan processing starts at the center of the hole, then moves rapidly to the hole perimeter and spins the beam for a programmed number of repetitions around the perimeter before returning rapidly to the center.
• Spiral processing starts at the center of the hole, moves rapidly to an inner diameter, then spins the beam positioner for a programmed number of revolutions, incrementing the diameter until the hole perimeter is reached.
• Laser beam movements may be carried out by a wide variety of laser beam positioning systems, such as the Model 53XX series of workpiece processing systems, manufactured by Electro Scientific Instruments, Inc., of Portland, Oregon, the assignee of this patent application.
• Prior tool patterns cause undue acceleration limits on positioner systems.
• Prior art trepanning entails moving the laser beam in a circular motion around the perimeter of the hole being processed. Skilled workers know that the radial acceleration of circular motion is equal to v 2 /R, where v is the tool velocity, and R is the radius of the circular motion.
• trepanning is preceded by an initial move segment that transitions in a smooth manner between the center of the hole and the start of circular motion to limit the tool acceleration and jerk (rate-of-change of acceleration).
• the acceleration required by the initial move segment is 2v 2 /R, which is twice the acceleration required for the circular motion.
• the motion axis requiring the double acceleration is the same axis executing a half-time duration acceleration pulse, resulting in a jerk profile four-times greater than circular motion requires.
• the laser beam positioning system acceleration is limited because twice the motor current is demanded at twice the servo frequency.
• Prior spiral tool patterns are limited to outward spiraling, which limits the types of material that can be processed.
• Prior trepan and spiral tools require time-wasting multiple steps for processing a hole with both spiral and repeated perimeter motions. Executing multiple steps requires the beam positioner to perform a generic move algorithm that requires at least two acceleration pulses to move the tool back to the center of the hole between steps.
• Prior trepan tool patterns may cause uneven removal of material. This is so because the laser beam energy is concentrated in one quadrant of the hole as the beam moves from the center of the hole to the perimeter, and back again.
• Prior spiral and trepan tools do not synchronize the timing of laser triggering signals with beam positioner motion, which causes an omission of a first hole processing pulse because typical Q-switched lasers do not generate a first pulse on command.
• Prior trepan tool patterns used for drilling holes with multiple repetitions at the perimeter substantially overlap laser pulses around the hole perimeter, and thereby cause uneven removal of material.
• workpiece processing machines having tool patterns that produce smaller, high- quality holes in a variety of workpiece materials, such as virtually any printed wiring board material, whether rigid or flexible, copper-clad or exposed, fiber reinforced, or homogeneous resin dielectric.
• the workpiece materials may also include ceramic substrates and silicon substrates, such as those employed in semiconductor devices.
• An object is, therefore, to provide a method of starting and ending circular drill motions with specifiable beam positioner accelerations.
• Another object is to provide a method for generating various new tool patterns.
• a further object is to provide a method for adjusting tool pattern parameters for achieving uniform removal of hole material.
• Still another object is to provide a method for controlling laser firing patterns and timing for performing workpiece processing.
• Yet another object is to provide a method for synchronizing laser firing with arbitrary tool positions on a workpiece.
• the following embodiments generate tool pattern movement commands for operating a laser beam positioner and for timing associated laser firing commands.
• the following aspects are identified by numbers that match the numbers identifying the corresponding problems set forth above as background information.
• Preferred tool patterns reduce beam positioner acceleration and jerk problems by approaching hole locations from outside the center of the hole.
• a move segment referred to as a dt/2 segment
• a duration equal to one-half of the circular segment duration and has zero acceleration.
• This approach movement results in much less servo error.
• the tool velocity is constrained by the square root of available acceleration, this hole approaching method allows tool velocity to be increased by 41 %, unless constrained by other factors.
• the removal of high acceleration from the dt/2 segment also allows the maximum circular oscillatory frequency to be increased while maintaining hole drilling quality.
• the ratio of the dt/2 segment acceleration to circular acceleration when drilling is defined as a factor a.
• the tool patterns support outward spiraling, inward spiraling, and combined outward and inward spiraling, all executed without turning laser pulsing off between move segments. Inward spiraling is often better for processing glass reinforced materials, such as nonhomogeneous glass reinforced etched-circuit board . material.
• the tool patterns can execute spiral and repeated perimeter processing in a single step without turning off laser pulsing.
• Positioner settling time is user programmable and is spent by the beam positioner tracing the circular path of the initial hole diameter to be processed, which tracing does not limit the beam positioner field.
• the above-described settling time improvement also causes settling to occur while the beam positioner is oscillating, so transients from slewing to oscillatory motion are spent settling rather than processing.
• the tool patterns employ an improved method for handling multiple repetitions of the tools. Rather than ending a circular motion with a generic move to set up the next repetition, this method maintains the osculation with a 90 degree phase difference between the positioner axes, but turns off laser pulsing.
• the prior method required a laser-off time between repetitions equal to one-quarter the revolution time of the initial repetition, plus a generic move time, plus one-quarter the revolution time of the next repetition.
• This method requires at most a one-quarter revolution time of the initial repetition, plus a minimum drill time (drill-Tmin), plus a one-quarter revolution time of the next tool repetition.
• Minimum drill-Tmin is less than the generic move Tmin because of the small motion employed.
• the required laser-off time is only one-quarter the revolution time of the initial repetition plus one-quarter the revolution time of the next repetition.
• a beam positioner and laser synchronization method schedules laser firing signals for firing a first laser pulse before the beam positioner reaches the target hole location, so that the second laser pulse, which is the first pulse actually fired, lands where desired and all pulses commanded thereafter are delivered to the workpiece.
• This method further includes a "fractional laser delay" parameter that is added into the half-sine profiler parameter set for turning the laser on in the middle of an acceleration segment.
• the tool patterns support an "incremental bite size" distribution of pulses on the hole perimeter that account for how many tool revolutions (repetitions) are executed at the perimeter. This optimizes the laser pulse distribution evenly and finely around the hole perimeter.
• the incremental bite size is defined as the distance along the perimeter between the first pulses delivered in the first and second revolutions (repetitions) of the tool.
• the incremental bite size method provides for automatically adjusting the tool velocity to set the incremental bite size to equal the laser bite size divided by the number of tool revolutions (repetitions).
• Fig. 1 A is a graph representing a first set of X- and Y-axis positions versus time of a beam positioner for directing a laser beam along a tool pattern.
• Fig. 1 B is an XY plot representing the entry, circular, and exit segment laser beam motions resulting from the first set of X- and Y-axis beam positions of Fig. 1A.
• Fig. 2A is a graph representing a second set of X- and Y-axis positions versus time of a beam positioner for directing a laser beam along a tool pattern.
• Fig. 2B is an XY plot representing the entry, circular, and exit segment laser beam motions resulting from the second set of X- and Y-axis beam positions of Fig. 2A.
• Fig. 3 is an XY plot representing an circular tool pattern generated by a laser beam positioning method.
• Fig. 4 is an XY plot representing an outward spiral tool pattern generated by a laser beam positioning method.
• Fig. 5 is an XY plot representing an inward spiral tool pattern generated by a laser beam positioning method.
• Fig. 6 is an XY plot representing an inward and outward spiral tool pattern generated by a laser beam positioning method.
• Fig. 7 is an XY plot representing two repetitions of an outward spiral tool pattern generated by a laser beam positioning method.
• Fig. 8 is a photograph of an unacceptable etched-circuit board via processed with multiple repetitions of a prior art trepan tool pattern employing a very small incremental bite size.
• Fig. 9 is a photograph of a high-quality etched-circuit board via processed with multiple repetitions of a circular tool pattern employing an incremental bite size chosen in accordance with a pulsed laser emission method.
• Fig. 10 is an XY plot representing a via processed by five repetitions of a prior art trepan tool pattern employing a prior art tool velocity.
• Fig. 11 is an XY plot representing a via processed by five repetitions of a circular tool pattern employing a calculated tool velocity.
• Fig. 12 is an XY plot representing laser pulses unevenly distributed around the perimeter of hole by two repetitions of a prior art trepan tool pattern.
• Fig. 13 is an XY plot representing laser pulses evenly distributed around the perimeter of hole by two repetitions of a circular tool pattern employing an "Equalize Perimeter Pulse Overlap" method.
• Fig. 14 is a simplified electrical block diagram representing register structures controlling and supporting the above methods.
• Figs. 15A and 15B are electrical waveform timing diagrams representing respective normal and special-case timing relationships for firing laser pulses supporting the above methods.
• Fig. 16 is an electrical waveform timing diagram showing timing relationships among laser beam positioning commands, various system delays, and the laser beam pulsing.
• positioner systems must control jerk, which is the rate of change of acceleration.
• jerk is the rate of change of acceleration.
• Many prior positioner systems effected circular motion with a string of short interconnected linear moves.
• the sudden angular change at each interconnection produced unacceptably large jerks that limited speed and positioning accuracy.
• Circular motion is fundamental in hole drilling applications, so employing sinusoidal positioner driving waveforms is preferred.
• preferred positioner driving waveforms employ half-sine-shaped acceleration segments that start and stop at a zero acceleration point.
• Each acceleration segment has a period TMIN that is the shortest non-zero acceleration half-sine segment within the acceleration capability of the positioner system and avoids positioner resonance problems.
• circular motion is achieved by driving the orthogonal axes of a positioner with a pair of 90-degree phase shifted sinusoidal waveforms.
• the X-axis is driven by two half-sine acceleration segments, and the Y- axis is driven by three 90-degree phase shifted half-sine segments.
• the X-axis has "filler" segments, referred to as entry and exit segments, for separating hole processing from movement between hole locations.
• a first aspect of this invention is, therefore, a method of starting and ending circular tool patterns with specifiable beam positioner accelerations on the dt/2 segments.
• Producing circular motion entails generating a pair of 90-degree phase-shifted sinusoidal motion waveforms for driving the beam positioner X and Y axes.
• Generating the 90-degree phase shift entails inserting a half-sine segment into one of the axes (which one depends on the tool pattern entry angle) with dt equal to one-half the dt of the circular motion segments.
• the phase-shifted segment is, therefore, referred to as the dt/2 segment. Users can specify the dt/2 segment acceleration from zero to twice the circular acceleration to trade off the initial beam position vs.
• FIG. 1 A and 1 B show a first set of X-axis positions 10 and Y-axis positions 12 versus time of a beam positioner (not shown) for directing a laser beam axis along a tool pattern beam path 14.
• Beam path 14 starts at a starting location 16 (shown as a dot), includes an entry segment 18, a 360-degree circular segment 20 (shown in dashed lines), an exit segment 22, and an ending location 24 (shown as a dot), which is also a center 25 of circular segment 20.
• Circular segment 20 has a diameter D, corresponds to the perimeter or periphery of a hole to be processed, and can have other than a 360-degree extent.
• Figs. 2A and 2B show a second set of X-axis positions 30 and Y-axis positions 32 versus time of the beam positioner for directing the laser beam axis along a tool pattern beam path 34.
• Beam path 34 starts at a starting location 36 (shown as a dot), includes an entry segment 38, a 360-degree circular segment 40 (shown in dashed lines), an exit segment 42, and an ending location 44 (shown as a dot).
• entry, segment 38 has a set to one
• exit segment 42 has a set to 0.5. Therefore, the acceleration of entry segment 38 is the same as the acceleration of circular segment 20, and the acceleration of exit segment 22 is one- half the acceleration of circular segment 20.
• the tool patterns employ entry segments, such as entry segments 18 and 38, and employ exit segments, such as exit segment 42.
• a series of holes is processed in a workpiece by directing a laser beam axis along a pathway that links together the ending location of a previous hole and the starting location of a next hole.
• the entry and exit segment method allows tool movement velocity to be increased up to 41 % over that achievable with prior methods.
• a second aspect of this invention provides a method for generating half- sine parameters, such as X- and Y-axis positions 10, 12, 30, and 32 of Figs. 1A and 2A, for executing various tool patterns.
• the parameters and method are described with reference to a Matlab code representation of the invention and several associated figures illustrating features of the tool patterns.
• Matlab is a model-based design simulation program that is available from The MathWorks of Natick, Massachusetts.
• the Matlab code for generating the move segments is set forth in Appendix A.
• FIG. 3 shows a circular tool pattern 50 generated by the method.
• Circular tool pattern 50 is employed for excising holes in a material by cutting around a periphery 51 of the hole being processed.
• each hole is processed with reference to degrees of rotation about an X-Y coordinate axis, where the +X, +Y, -X, and -Y axes are oriented, respectively at 0, 90, 180, and 270 degrees.
• Circular tool pattern 50 includes an entry segment 52 having a starting location 54 at 270-degrees and an entry location 56 at 0-degrees, where laser pulses 58 are initiated.
• the hole being processed has a 125 //m diameter, and laser pulses 58 have a 20 ⁇ m effective spot size.
• a 10.25//m laser bite size results in 33 of laser pulses 58 being distributed within periphery 51 during a single 360-degree repetition of tool pattern 50 starting and ending at entry location 56.
• Laser pulses 58 are turned off and tool pattern 50 follows an exit segment 60 to an ending location 62 at 90-degrees.
• entry and exit segments 52 and 60 represent merely one exemplary set of relative angles that may be offset about the X and Y axes depending on the relative locations of prior and subsequent holes to be processed.
• entry segment 52 may start and end at 0- and 90-degrees and, therefore, exit segment 60 may start and end at 90- and 180-degrees.
• Typical positioner, laser, and hole parameters associated with tool pattern 50 include a tool velocity of 717 mm/sec, a laser pulse repetition frequency ("PRF") of 70 KHz, a positioner maximum acceleration of 1 ,000 Gs, a via drilling time of 0.47 msec, and a via minimum move time of 0.7 msec, resulting in a maximum via processing rate of 855 vias/sec.
• PRF laser pulse repetition frequency
• FIG. 4 shows an outward spiral tool pattern 70 generated by the method.
• Outward spiral tool pattern 70 is employed for drilling holes in a material by ablating material from center 25 along a curvilinear path progressively away from center 25 toward periphery 51 of each hole being processed.
• Outward spiral tool pattern 70 includes an entry segment 72 having a starting location 74 at 270-degrees and an entry location 76 at 0-degrees, where laser pulses 78 are initiated.
• the hole being processed has a 125 //m diameter
• laser pulses 78 have a 20 ⁇ m effective spot size and a 4.47 ⁇ m laser bite size.
• Tool pattern 70 There are 89 laser pulses 78 distributed during a single repetition of tool pattern 70 starting at entry location 76, spiraling outward for one 360-degree rotation, and processing within periphery 51 from 0-degrees to about 216-degrees, at which point laser pulses 78 are turned off to prevent overlap with previously processed locations.
• Tool pattern 70 follows an exit segment 80 having a starting location 82 at 270-degrees to an ending location 84 at 0-degrees.
• entry and exit segments 72 and 80 represent merely one exemplary set of relative angles that may be offset about the X and Y axes depending on the relative locations of prior and subsequent holes to be processed.
• entry segment 72 may start and end at 0- and 90-degrees and, therefore, exit segment 80 may start and end at 0- and 90-degrees.
• Typical positioner, laser, and hole parameters associated with tool pattern 70 include a tool velocity of 313 mm/sec, a laser PRF of 70 KHz, a positioner maximum acceleration of 1 ,000 Gs, a via drilling time of 1.27 msec, and a via minimum move time of 0.99 msec, resulting in a maximum via processing rate of 442 vias/sec.
• FIG. 5 shows an inward spiral tool pattern 90 generated by the method.
• Inward spiral tool pattern 90 is employed for drilling holes in a material by ablating material from periphery 51 along a curvilinear path progressively inward from periphery 51 toward center 25 of each hole being processed.
• Inward spiral tool pattern 90 includes an entry segment 92 having a starting location 94 at 90-degrees and an entry location 96 at 180-degrees, where laser pulses 98 are initiated.
• the hole being processed has a 125 ⁇ m diameter
• laser pulses 98 have a 20 ⁇ m effective spot size, and a 4.47 ⁇ m laser bite size.
• Tool pattern 90 There are 273 laser pulses 98 distributed during a single repetition of tool pattern 90 starting at entry location 96, processing two and one-half revolutions (900-degrees) within periphery 51 starting at 180-degrees and ending at 0-degrees, spiraling inward for two 360- degree revolutions starting and ending at 0-degrees, at which point laser pulses 98 are turned off.
• Tool pattern 90 follows an exit segment 100 having a starting location 102 at 0-degrees to an ending location 104 at 90-degrees.
• entry and exit segments 92 and 100 represent merely one exemplary set of relative angles that may be offset about the X and Y axes depending on the relative locations of prior and subsequent holes to be processed.
• entry segment 92 may start and end at 270- and 0-degrees and, therefore, exit segment 100 may start and end at 90- and 180-degrees.
• Typical positioner, laser, and hole parameters associated with tool pattern 90 include a tool velocity of 313 mm/sec, a laser PRF of 70 KHz, a positioner maximum acceleration of 1 ,000 Gs, a via drilling time of 3.9 msec, and a via minimum move time of 0.85 msec, resulting in a maximum via processing rate of 211 vias/sec.
• FIG. 6 shows an inward and outward spiral tool pattern 110 generated by the method.
• Inward and outward spiral tool pattern 110 is employed for drilling holes in a material by ablating material from periphery 51 inward to the center of each hole being processed and back outward to periphery 51.
• Inward and outward spiral tool pattern 110 includes an entry segment 112 having a starting location 114 at 180- degrees and an entry location 116 at 270-degrees, where laser pulses 118 are initiated.
• the hole being processed has a 125 ⁇ m diameter
• laser pulses 118 have a 20 ⁇ m effective spot size and a 4.47 ⁇ m laser bite size.
• Tool pattern 110 There are 132 laser pulses 118 distributed during a single repetition of tool pattern 110 starting at entry location 116, processing one-quarter revolution (90-degrees) within periphery 51 starting at 270-degrees and ending at 0-degrees, spiraling inward for one 360-degree revolution starting and ending at 0-degrees, processing one-half revolution from 0-degrees to 180 degrees about the center of the hole, spiraling outward for one 360-degree revolution starting and ending at 180-degrees, and processing one-quarter revolution from 180-degrees to 270-degrees within periphery 51 , at which point laser pulses 118 are turned off.
• Tool pattern 110 follows an exit segment 120 having a starting location 122 at 270-degrees to an ending location 124 at 0-degrees.
• angular locations of entry and exit segments 112 and 120 represent merely one exemplary set of relative angles that may be offset about the X and Y axes depending on the relative locations of prior and subsequent holes to be processed.
• Typical positioner, laser, and hole parameters associated with tool pattern 110 include a tool velocity of 313 mm/sec, a laser PRF of 70 KHz, a positioner maximum acceleration of 1 ,000 Gs, a via drilling time of 1.88 msec, and a via minimum move time of 1.03 msec, resulting in a maximum via processing rate of 434 vias/sec.
• Fig. 7 shows two repetitions of outward spiral tool pattern 70 (two repetitions referred to as 70') generated by the method.
• Tool pattern 70' is shown with half-sized laser spots to more clearly show the entry and exit segment trajectories.
• Outward spiral tool pattern 70' is employed for drilling holes in a material by repeatedly ablating material from the center outward to periphery 51 of each hole being processed.
• Outward spiral tool pattern 70' includes an entry segment 72' having a starting location 74' at 270-degrees and an entry location 76' at 0-degrees, where laser pulses 78' are initiated.
• the hole being processed has a 200 ⁇ m diameter
• laser pulses 78 have a 10 ⁇ m effective spot size and a 4.47 ⁇ m laser bite size.
• the second repetition of the tool pattern 70' starts at transition ending location 84', spiraling outward for one 360-degree rotation to 180-degrees, and processing within periphery 51 from 180-degrees to 270- degrees, at which point laser pulses 78' are turned off again and tool pattern 70' follows an exit segment 80" having a starting location 82" at 270-degrees to an ending location 84" at 0-degrees.
• angular locations of entry and exit segments 72 and 80 represent merely one exemplary set of relative angles that may be offset about the X and Y axes depending on the relative locations of prior and subsequent holes to be processed.
• Typical positioner, laser, and hole parameters associated with tool pattern 70' include a tool velocity of 313 mm/sec, a laser PRF of 70 KHz, a positioner maximum acceleration of 1 ,000 Gs, a via drilling time of 3.08 msec, and a via minimum move time of 1.69 msec, resulting in a maximum via processing rate of 209 vias/sec.
• a third aspect of this invention provides a method for adjusting the laser beam movement velocity to achieve a uniform laser energy distribution while processing holes.
• processing good quality holes depends on slightly overlapping the laser spots of each subsequent repetition.
• the degree of overlap from repetition to repetition is determined by a parameter referred to as "incremental bite size.”
• the incremental bite size is defined as the distance that the laser pulse locations shift between repetitions of a circular tool pattern.
• Good via processing depends on shifting the locations of the laser pulses slightly for each repetition of the circular tool pattern so that laser pulses do not hit the same spot during subsequent repetitions and the laser energy is more uniformly spread around the via periphery.
• the laser pulses shift for each repetition so that a hypothetical sixth repetition has pulses that exactly overlap the first repetition pulses (the incremental bite size is approximately equal to the bite size divided by the number of circular repetitions).
• poor via processing results when the pulses from each repetition impinge on the same locations. This is typically caused by inadvertently employing an incremental bite size that is very small relative to or approximately equal to the actual bite size.
• Figs. 8 and 9 show respective unacceptable and high-quality vias processed in etched-circuit board material by multiple repetitions of trepan and circular tool patterns employing, respectively, a prior art bite size and a preferred calculated incremental bite size.
• Figs. 10 and 11 show how the incremental bite size is affected by small changes in laser beam velocity.
• Fig. 10 shows a prior art bite size resulting by employing five repetitions of a trepan tool pattern in which the laser PRF is 30 kHz, the via diameter is 125 /ym, the effective laser beam spot size is 13 //m, and the laser beam velocity is 377.5 mm/s.
• Fig. 10 shows that the laser pulses from each of the five trepan repetitions almost exactly overlap, resulting in a poor-quality via and/or poor trepan processing robustness.
• Fig. 11 shows the same process except that the laser beam velocity is changed slightly to 379.5 mm/s.
• the small 2 mm/sec velocity change causes a uniform distribution of laser pulses about the via periphery, resulting in a high-quality via and/or improved process robustness.
• the circular process of Fig. 11 employed a 2.3 ⁇ m incremental bite size, which causes laser pulses from the hypothetical sixth repetition to overlap laser pulses from the first repetition.
• the laser beam velocity needed for properly setting the incremental bite size associated with a particular circular tool pattern depends on the number of repetitions employed, the via diameter, the laser PRF, and the effective laser beam spot size.
• the laser beam velocity is preferably chosen such that the laser beam pulse locations of the first and hypothetical last plus one tool pattern repetitions substantially overlap.
• PRF Laser pulse repetition rate in kHz
• N rep Number of pulses in one circular repetition
• EJf Effective spot size in ⁇ m
• a rep Incremental bite size in ⁇ m
• Appendix B sets forth a Matlab coded method, based on equations Eq. 1 to Eq. 3, for adjusting the incremental bite size to achieve equalized pulse spacing when employing multiple repetitions of a circular tool pattern.
• the method is executed when a user actuates an "Equalize Perimeter Pulse Overlap" button or other actuator.
• the method adjusts the tool velocity and, therefore, the bite size downward a small amount to achieve the desired incremental bite size without significantly changing the laser pulse energy density.
• the incremental bite size can be adjusted by changing any combination of the tool velocity, PRF, and effective spot size, which corresponds to changing the hole diameter. Therefore, a more rigorous mathematical description of incremental bite size is set forth below. [0086]
• the effective hole diameter is defined by:
• a quantity "x" is defined by Eq. 8:
• Equation (9) has an infinite solution set in which any positive number with
• the incremental bite size can be effected by altering the velocity, PRF, diameter, or effective spot size, according to equation 8. Solving for incremental bite size by adjusting the velocity is shown below in equation 12:
• the effective diameter can be altered by changing a combination of the hole diameter and the effective spot size.
• the incremental bite size is preferred determined by employing equation 12 for adjusting the beam positioner velocity.
• Figs. 12 and 13 show a comparative relationship between trepan and circular tool patterns formed in accordance with, respectively, the prior art and this method.
• Fig. 12 shows laser pulses 130 unevenly distributed around a periphery 132 of a hole by two repetitions of a prior art trepan tool pattern 134.
• Trepan tool pattern 134 employs a 717 mm/sec laser beam velocity, a 70 kHz laser PRF, a 125 //m hole diameter, a 20 ⁇ m effective spot size, a 10.24//m laser bite size, and an 8.15 /ym bite size 136.
• FIG. 13 shows laser pulses 130 evenly distributed around periphery 132 of the hole by two repetitions of a circular tool pattern 138 employing an "Equalize Perimeter Pulse Overlap" method.
• Circular tool pattern 138 employs a 710.5 mm/sec laser beam velocity, a 70 kHz laser PRF, a 125 ⁇ m hole diameter, a 20 ⁇ m effective spot size, a 10.15//m laser bite size, and a 5.07 ⁇ m incremental bite size 139.
• Employing the "Equalize Perimeter Pulse Overlap" method changes the laser beam velocity from 717 mm/sec to 710.5 mm/sec.
• a fourth aspect of this invention provides a method for controlling a Q-switched laser that emits laser beam pulses employed by the above-described tool patterns.
• the method executes on a logic apparatus, such as a microprocessor of a digital signal processor ("DSP") including registers for controlling a field programmable gate array (“FPGA”) that precisely schedules emission of a first laser pulse and emissions of all subsequent laser pulses relative to half-sine profiler commands driving a laser beam positioner.
• DSP digital signal processor
• FPGA field programmable gate array
• Exemplary profiler commands may include commands required to position a laser beam along the tool pattern segments described above.
• Fig. 14 shows a first DSP register, which is a "Laser Rep Rate Register” 140 that includes 16 read/write locations for programming the laser interpulse period.
• Laser Rep Rate Register 140 controls a rep rate generator 142 that provides a signal having a frequency fn ranging from 305.18 Hz (register 140 value OxFFFF) to 2.0 MHz (register 140 value 0x0) with a resolution of about 0.0047 Hz.
• the signal frequency fn 1/( reg. value * 50 nsec).
• Rep rate generator 142 is activated only when a DSP "Pulse Count Register" 144, described below, stores a non-zero value.
• a third DSP register is Pulse Count Register 144 that includes 18 read/write locations for programming a number of laser pulses that will be emitted in a burst of pulses.
• Valid register 144 values range from 0x3FFFF to 0. Values 0x3FFFF and 0x0 have special meanings. Valid values (1 through 0x3FFFE, 262142D) correspond to the number of pulses emitted in the burst.
• Special value 0x3FFFF causes a continuous burst of pulses until a zero value is written to Pulse Count Register 144.
• Special value 0x0 stops the current burst.
• the method for initiating emissions of laser pulses is described below with reference to Figs. 15A and 15B. Laser pulse commands are qualified by a watch ⁇ dog timer that stops emission of laser pulses if the DSP fails. Skilled workers will understand how to implement the logic apparatus for carrying out this laser control method.
• Figs. 15A and 15B show respective normal and special-case timing relationships for emitting laser pulses supporting the above methods.
• the laser PRF is set by the DSP writing an interpulse period 150 to Laser Rep Rate Register 140.
• Pulse Count Register 144 is initialized with a value of zero.
• the DSP initiates the laser pulsing process with a "DSP_Write_Strobe_N" write signal 152, which loads a number of pulses value (1 to 200,000) 154 into Laser Pulse Count
• An "FPGA_Gate_N" laser gate signal 156 goes true after an amount of gate delay (50 ns to 100 ⁇ s) 158 as determined by bits 00 to 10 of Laser Pulse
• a first "FPGA_QSW_N" laser Q-Switch signal 160 goes true after an amount of first pulse delay (50 ns to 100 //s) 162 as determined by bits 12 to 22 of
• the default first pulse delay 162 is zero.
• Laser gate signal 156 goes false 50 ns after a last laser pulse 164 causing a last laser pulse emission goes true.
• the special timing case occurs when DSP write signal 152 loads a value 0x3FFFF into Pulse Count Register 144. This action causes a pulse counter 148 to count continuously until a DSP write signal 152' loads a zero value into Pulse Count Register 144, which causes laser Q-switch signal 160 pulses to stop after a one pulse uncertainty delay 166 to avoid race conditions plus the remainder of gate delay 158 and first pulse delay 162.
• a fifth aspect of this invention entails a method for coordinating the emission of laser pulses and their incidence on predetermined laser beam positioning command locations.
• the emitted laser pulses are precisely positioned during a desired number of tool pattern repetitions and, in particular, to emit the first laser pulse of each repetition at the correct location of the tool pattern. Therefore, this method improves the coordination, accuracy, and performance of the motion profiler and laser timing employed by the above-described tool patterns.
• This method supports the new tool patterns by providing a higher tool velocity for a given laser beam positioner acceleration limit. For example, a typical galvanometer-based beam positioner has a 1 ,000 G acceleration limit.
• the new tool patterns affect laser pulse emission timing in at least two ways.
• the circular tool pattern can start laser pulse emission in the middle of a move segment and allows laser pulsing during fractional portions of a tool repetition. Therefore, a portion of the laser beam positioner system, referred to as a coordinated motion control module ("CMCM"), coordinates with the system control computer to cause laser pulse emissions during predetermined fractions of a move segment.
• CMCM coordinated motion control module
• the new mid-segment laser timing coupled with tool velocities approaching 1.0 m/sec requires very high laser pulse timing accuracy.
• Prior laser timing systems have about a ⁇ 50 //sec pulse first pulse timing resolution, which implies an unacceptable ⁇ 50 ⁇ m first pulse positioning.
• this method transfers the precise timing of laser pulses from DSP control to the much faster FPGA by transferring the contents of the DSP registers and related timing control to counterpart registers 140', 144', and 146' in the FPGA.
• a new FractionalLaserDelay parameter is added to the move segment data structure for coordinating CMCM motion commands and laser pulse emission timing.
• the FractionalLaserDelay parameter defines a time delay between the start of a move segment and the first laser pulse emission, as a fraction of the total segment time ⁇ T.
• the FractionalLaserDelay parameter has an 8-bit value, with values from 0 to 255. If the value is zero, laser pulse timing behaves like the prior art.
• the delay from the start of a move segment to the first laser pulse is:
• Delay ⁇ T ⁇ ractionalLaserDelay/256.
• Fig. 16 shows the laser beam pulsing and beam positioner ("BP") timing relationships.
• emission of a first laser pulse 170 would occur a FractionalLaserDelay* ⁇ T time 172 after the beam positioner starts a tool pattern entry segment, such as entry segment 52 (Fig. 3).
• a number of system delays require a coordinated timing method for emission of an actual first laser pulse 174.
• the coordinated timing method first accounts for a CoordinatedModeFilterDelay 176 that includes a profiling filter group delay and a galvanometer delay.
• the profiling filter group delay has a fixed value of 50-80 msec, depending on the filter frequency.
• Coordinated mode beam positioning and the associated group filter delay is described in U.S. Pat. No. 5,751 ,585 for HIGH SPEED, HIGH ACCURACY MULTI-STAGE TOOL POSITIONING SYSTEM, which is assigned to the assignee of this application.
• the GalvoDelay is the time required for the beam positioner command to reach the beam deflecting galvanometers. GalvoDelay is fixed at about 200 msec.
• the coordinated timing method further accounts for a LaserEventBufferDelay 178 that includes the elapsed time between a profiled drill segment, such as entry segment 52 (Fig. 3), and the time commanded to start laser emission.
• LaserEventBufferDelay 178 can be adjusted to a resolution of 50 msec.
• GateDelay 158 (also see Figs. 15A and 15B) is stored in a first portion of FPGA Pulse Control Timer Register 146' and determines the FPGA delay between receiving the Pulse Count write signal 152 and activating laser gate signal 156 (also see Figs. 14A and 14B).
• GateDelay 158 has a 50 nsec resolution.
• FirstPulseDelay 162 (also see Figs. 14A and 14B) is stored in a second portion of FPGA Pulse Control Timer Register 146' and determines the delay between laser gate signal 156 and first laser Q-switch signal 160 for requesting emission of the first actual laser pulse.
• FirstPulseDelay 162 is determined by 1/PRF + RuntDelay, where RuntDelay is a fixed delay required to avoid an abnormally low energy (runt) laser pulse.
• CoordinatedModeFilterDelay+GalvoDelay is stored as a parameter named BPDelay.
• Delayi BPDelay + ⁇ T*FractionalLaserDelay - FirstPulseDelay. This is the delay required between LaserEventBufferDelay 178 and GateDelay 158.
• GateDelay 158 (Delayi modulus 50 msec) + 50 msec. This delay ensures that GateDelay 158 is long enough to avoid FPGA boundary conditions.
• LaserEventBufferDelay 178 Delayi - GateDelay 158. This is an even multiple of 50 msec.
• GateDelay 158 is loaded as a field in a LaserOn packet of the LaserEventBuffer, and a time tag for the packet is the current time plus LaserEventBufferDelay.
• the beam positioner servo calls a LaserOn packet, it loads the GateDelay 158 value into FPGA Laser Pulse Control Timer Register 146', then interrogates FPGA Pulse Count Register 144' to determine the desired number of laser pulses.
• workpiece specimen materials may include virtually any printed wiring board material, whether rigid or flexible, copper-clad or exposed, fiber reinforced, or homogeneous resin dielectric, and may also include ceramic substrates and silicon substrates, such as those employed in micro-electronic and semiconductor devices.
• the bite size microns.
• the "incremental" bite size which is the advance of the first pulse of the 2nd revolution relative to the 1st revolution set (handles.bite_size, 'String 1 , nutr ⁇ str(velocity*prf_period) ) ;
• imag_bite_size mod(prf_period-mod(Trev,prf_jperiod) ,prf_period) *velocity; set (handles.imag_bite_size, 'String' , num2str(i ⁇ nag_bite_size) ) ;

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